This invention relates to apparatus for controlling ac servo motors of synchronous type.
There are several control systems or methods for controlling ac servo motors of synchronous type depending on the usage, and a control system using rectangular wave current is advantageous in view of cost in the case where relatively high accuracy is not required.
FIG. 13 shows a conventional apparatus for controlling an ac servo motor. The apparatus comprises a dc power source 1 and an inverter 2 comprising a plurality of power transistors and a plurality of diodes respectively connected to the transistors in inverse parallel. These transistors of the inverter 2 are controlled by a base drive unit 11. The reference 3 designates a synchronous motor driven by the inverter 2, and the reference 4 designates a rotor position sensor for detecting the rotational position of the rotor (not shown) of the motor 3. The reference 5 is a rotary encoder for detecting the rotational speed of the motor 3. The reference 6 is a speed-to-voltage converter responsive to an output pulse signal indicative of the motor speed from the rotary encoder 5 for converting a speed signal into a voltage. The reference 7 is a speed amplifier for amplifying the difference between a command or setting speed and a detected speed from the speed-to-voltage converter 6. The reference 8 is a current distribution circuit for controlling current distribution using an output signal from the rotor position sensor 4 so that current is distributed to stator windings to be energized. The reference 9 is a multiplier for commanding current distribution of phases U, V and W of the stator windings in accordance with output signals from the current distribution circuit 8. The reference 10 is a combination of a current amplifier and a PWM circuit for flowing command or setting currents in the respective phases through comparison between respective detected currents of the respective phases U, V and W and their command currents. The base drive unit 11 is responsive to the output signals from the combination 10 of the current amplifier and the PWM circuit.
Although the above-described ac servo motor control apparatus is more economical than apparatus performing sinusoidal wave driving, it requires at least two current detectors, where these current detectors are required to isolate dc voltage for current detection. Therefore, the apparatus is generally expensive in cost. Thus, the conventional structure is not suitable for simple servo control apparatus while less cost apparatus is desired.